1. Field of the Invention
The present invention relates to a fuel cell stack and a method of operating the fuel cell stack. The fuel cell stack comprises a plurality of fuel cells stacked together. Each of the fuel cells includes a pair of separators and an electrolyte electrode assembly interposed between the separators. The fuel cell stack further comprises an anode current collector and a cathode current collector stacked on outermost fuel cells at opposite ends in a stacking direction of the fuel cells.
2. Description of the Related Art
In recent years, various types of fuel cells such as a polymer electrolyte fuel cell (PEFC) have been developed. The polymer electrolyte fuel cell employs a membrane electrode assembly (MEA) which comprises two electrodes (anode and cathode) and an electrolyte membrane interposed between the electrodes. The electrolyte membrane is a polymer ion exchange membrane (proton exchange membrane). Each of the electrodes comprises a catalyst and a porous carbon. The membrane electrode assembly is interposed between separators (bipolar plates). The membrane electrode assembly and the separators make up a unit of the fuel cell for generating electricity. A plurality of fuel cells are connected together to form a fuel cell stack.
In the fuel cell, a fuel gas such as a hydrogen-containing gas is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions (protons) and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electric current. An oxygen-containing gas or air is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to form water.
In the fuel cell stack, temperature of some of the fuel cells tends to be low in comparison with the other fuel cells. Specifically, one end surface of each of the outermost fuel cells (end cells) in the stacking direction is exposed to the external air, and thus, the end cells are likely to be cooled by the external air. If the temperature of the end cells is lowered significantly, the power generating performance of the end cells is lowered. Further, dew condensation may occur in the end cells. Water produced in the chemical reaction is not smoothly discharged from the end cells, and the voltage of the end cells are lowered.
In particular, when the fuel cell stack is operated at a temperature below the freezing point, the difference between the temperature in the fuel cell stack and the external air temperature is large. Therefore, the temperature in each of the end cells is lowered significantly. If the operation the fuel cell stack is started at the temperature below the freezing point, water produced at the time of power generation in each of the end cells is cooled down below the freezing point. The frozen water may close reaction gas flow passages (oxygen-containing gas flow passage and/or fuel gas flow passage) or the porous carbon undesirably. As a result, a shortage of reaction gases may occur in the end cells. The shortage of the reaction gases gives rise to a voltage drop in the end cells.
In an attempt to prevent the end cells from being cooled excessively, for example, Japanese laid-open patent publication No. 8-130028 (the prior art 1) discloses a solid polymer electrolyte fuel cell stack which does not have any grooves (coolant passages) in outer separators of end cells for preventing the separators from being cooled excessively.
Further, Japanese laid-open patent publication No. 8-167424 (prior art 2) discloses a solid polymer electrolyte fuel cell stack which includes heating members heated by an electric current flowing from the solid polymer electrolyte fuel cell stack. The heating member is disposed at least on each of current collectors in contact with the outer surfaces of outermost separators of the fuel cell stack for preventing end cells from being cooled excessively.
In the prior art 1, the solid polymer electrolyte fuel cell stack requires different types of separators, i.e., the separator which has the groove as the coolant passage, and the separator which does not have the groove. The requirement for the different types of separators is a burden in the production line, and thus, the production cost is high.
In the prior art 2, the heating member is disposed between the current collector for collecting electricity and the fuel cell. The heating member constantly consumes electricity generated by the solid polymer electrolyte fuel cell stack. Even if the heating of the end cells is not necessary in the operation of the solid polymer electrolyte fuel cell stack, the heating member consumes electricity wastefully. Therefore, the power generation efficiency of the overall fuel cell stack is lowered.